The present invention relates to a pneumatic tire, more particularly to a studless tire improved in wet grip performance.
In recent years, studless tires or winter tires having no spike have come in to wide use.
In Japanese Patent No. 2637887, a rubber compound including organic fibers which is suitably used in the studless tires as tread rubber is disclosed, wherein, in order that the fibers are not oriented in a specific direction during extruding the raw compound, the organic fibers have a diameter of 0.1 to 0.3 mm and a length of 0.5 to 3 mm and the ratio of the diameter to the length is set in the range of from 0.06 to 0.6. And the content of the fibers is set in the range of from 5 to 20 parts by weight with respect to 100 parts by weight of material rubber compound. Therefore, in the tread portion of the finished tire, the fibers are not oriented in a specific direction and the tire tread rubber do not display directional characteristics.
In order to improve wet grip performance, especially on the ice covered road, of the above-mentioned studless tires, the present inventors studied and found that the road grip becomes maximum when all the fibers are oriented in the tire radial direction.
It is therefore an object of the present invention to provide a studless tire in which the tread rubber in the ground contacting region contains short fibers oriented in the radial direction of the tire.
According to the present invention, a studless tire comprises a tread portion,
the tread portion made of a vulcanized tread rubber,
the tread rubber compounded from 100 parts by weight of rubber component and 2 to 30 parts by weight of short fibers,
the tread portion provided in a ground contacting region with sipes
the sipes being formed by pressing thin plates onto the tread rubber during vulcanizing the tread rubber, whereby the thin plates orient the short fibers in a radial direction of the tire.
Preferably, the short fibers have a diameter of not more than 30 xcexcm and a length of from 0.3 to 20 mm, and the thickness of the thin plates or the width of the sipes is in the range of from 0.2 to 0.5 mm. The spacings between the sipes are less than 10 mm.
Further, the short fibers are generally oriented in the circumferential direction of the tire before pressing the thin plates, and the total length in millimeter of axial component of all the sipes being in the range of from 0.05 to 0.15 times the area in square millimeter of the ground contacting region.
An embodiment of the present invention will now be described in detail in conjunction with the accompanying drawings.